In recent years fiber optic gyroscopes (FOGs) have become widely used in several technologies to sense the rotation and angular orientation of various objects, such as aerospace vehicles. A FOG typically includes an optical fiber, often several kilometers in length, wound in a coil about an axis of rotation (i.e., the rotation to be sensed). Light is injected in opposite directions through the coil and directed onto a photo-detector. If the coil is rotated about the axis, the effective optical path length for the light traveling in one direction in the coil is increased, while the path length is decreased for the light traveling in the opposite direction.
The difference in path length introduces a phase shift between the light waves traveling in opposite directions, known as the Sagnac Effect. As a result, an interference pattern is detected by the photo-detector, which indicates that the FOG is experiencing rotation. The output signal from the photo-detector typically follows a cosine function. That is, the output signal depends on the cosine of the phase difference between the two waves. Therefore, because the cosine function is even, the rate of change near zero is very small, resulting in poor sensitivity for low rotation rates. To improve sensitivity, the waves are often modulated by a particular voltage to generate a phase difference deviating from zero. The actual phase generated by the modulation voltage is referenced to a voltage Vπ that changes the light phase by 180° (or π radians). The phase modulator reference voltage Vπ depends on the wavelength and environmental factors, such as temperature, humidity, and pressure.
The amplitude of the Sagnac phase shift caused by a rotation rate determines the FOG “scale factor,” which numerically relates the detected phase difference to the actual rotation being experienced by the FOG. As the fiber sensing coil length and diameter are increased, the scale factor also increases, and as a result, the FOG is more sensitive to rotation. The FOG scale factor also depends on wavelength of the light propagating in the fiber sensing coil. Due to imperfections in the light source and atrophy in the components of the FOG as well as environmental variables, the exact wavelength of the light being used is not known. Therefore, the exact scale factor can not be determined, and thus the FOG does not operate accurately.
Accordingly, it is desirable to provide a method for calibrating a fiber optic gyroscope scale factor by accurately measuring the wavelength of the light being used and environmental contributions of the phase modulator reference voltage Vπ. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.